Stabilization of polypropylene



United States Patent 3,462,291 STABILIZATION 0F POLYPROPYLENE Willard P. Conner, Chadds Ford, Pa., assignor to Hercules Incorporated, a corporation of Delaware No Drawing. Filed Jan. 20, 1966, Ser. No. 521,824 Int. Cl. D06m /70; C08f /58 U.S. Cl. 11766 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process for improving the laundry durability of stereoregular polypropylene fabrics.

In recent years, stereoregular polypropylene has commanded an increasingly larger share of the synthetic fiber market. Thanks to its unique combination of properties, it is an excellent material for many textile applications, ranging from carpeting and upholstery to fine wearing apparel.

In its natural state, polypropylene is extremely sensitive to oxidation, degrading rapidly when heated in air to temperatures in excess of about 100 C. Much research effort has been expended in finding effective stabilizers and stabilizing systems which will overcome this difficuty. These efforts have been highly successful and a great number of useful stabilizer systems have been developed, so that, generally speaking, heat stability of the polymer is satisfactory for most applications.

It has been noted, however, that the heat stability of polypropylene fabrics which are subjected to frequent laundering tends to decrease as the number of launderings increases. Analyses indicate that the stabilizer content of the polymer decreases with frequency of laundering. This is apparently due to volatilization of the stabilizer or to a leaching out of the same from the polymer by the water and the laundering chemicals. This reduction of the stabilizer content not only lessens the heat stability of the fabric but is also accompanied by a decrease in the tensile strength of the fibers and eventual failure thereof with holes forming in the fabric.

In accordance with this invention, it has now been found that the laundering durability of polypropylene fabrics can be substantially improved by adding a stabilizer in two or more steps, viz (1) during manufacture of the fiber from which the fabric is prepared and, (2) during one or more laundry treatments.

A standard commercial laundering cycle normally comprises a suds wash, a bleach, and a sour rinse, each step being followed by a hot water rinse. After the sour rinse, the fabric is dried in a drier at a temperature around 300 F. Most likely, the loss takes place periodically throughout the entire cycle. After each wash-dry cycle, the concentration of the antioxidant in the fabric is decreased.

The use of the three baths specified, followed by a dry step, is usually regarded as being essential to a complate and effective laundering job. The suds bath, of course, is self-explanatory, being the surface dirt removal step. This can comprise any efficient soap or detergent, such as glyceride soap or a long chain alkyl sulfonate, containing the usually wetting agents or other auxiliary components. The bleaching step neutralizes and removes residual soap or detergent and restores the original whiteness of the fabric. The sour rinse step neutralizes any residual bleach left in the fabric from the bleach step,

3,462,291 Patented Aug. 19, 1969 thereby preventing its exerting a deleterious effect upon the color of the fabric upon subsequent heating thereof as in drying and ironing. The sour rinse can also include a germicide for health reasons.

The composition of the various baths is subject to considerable variety, depending upon the specific job to done and the preferences of the laundere-r. The compositions of the baths are well known and form no part of the instant invention; the compositions not being critical to the efiicacy of the stabilizer addition technique of the invention. The significance or criticality of the laundry baths is that all known laundering techniques employing the suds wash, bleach, and sour rinse as described exhibit the same deleterious effect on stereoregular polypropylene fabric.

Experience indicates that, no matter where the extraction of stabilizer from the fiber might be considered to take place, the failure of the fabric occurs when it is heated during subsequent drying and ironing. Thus, the stabilizer addition technique herein described is efficacious so long as the second stabilizer addition to the polymer is effected prior to the accomplishment of these latter steps. To do this, it is possible to add the stabilizer in any of the washing steps. However, to be most efficient and to guard against undue further loss due to rinsing, the addition is preferably accomplished during the sour rinse step. This way, very little of the added stabilizer is lost to subsequent rinsing and very efficient stabilization is accomplished.

Normally, upon exposure to the above severe, commercial laundry procedure or a similar procedure, a polypropylene fabric fails after about 6 or 7 cycles. Failure is evidenced by a loss of tensile strength of the fabric and the development of holes therein following the drying. By using the stabilizer addition technique of the instant invention, it is possible, in many cases, to increase the service life of the fabric by and more, as will be shown in the following examples.

Additionally, and quite surprisingly, it has been found that a significant improvement in stabilizing effect per unit of stabilizer can be accomplished by proceeding according to the method of this invention. That is to say. longer fabric service life can be experienced with less stabilizer when stabilizer is added as herein described than is experienced when all the stabilizer is incorporated into the polymer during preparation thereof. This fact will be demonstrated conclusively in the examples which will be set out below. The reason for this increased efficiency of stabilizer utilization is not known for certain, but may be due to the fact that, when the procedure of the invention is employed, the bulk of the stabilizer is present on the surface of the individual filaments where it is readily available to perform its stabilizing function. In normal washing, however, the stabilizer is easily depleted from the surface. But since most polypropylene stabilizers are only sparingly soluble in water, the presence of a small amount of stabilizer in the laundry baths provides a buffering effect which decreases the rate of extraction of the stabilizer from the polymer. In view of the high cost of most polypropylene stabilizers, this greater efiiciency is a particularly useful advantage of the invention.

The method of the invention is applicable to all of the commonly known stabilizer systems for polypropylene. Exemplary, but not all inclusive of such systems, are the thio-bis-phenols, the alkylidene bis-phenols, thiodialkanoic acids and esters thereof and the phenolic-modified triazines. All of these stabilizers are subject to the previously mentioned deficiency of being poorly resistant to laundering, and all can be improved by the procedure herein described. The stabilizer added during laundering can be either the same as was used in the polymer originally or a different one can be used. Two or more stabilizers can be added, if desired.

The invention will now be described in more detail by means of several specific examples.

The standard washing procedure set forth previously was employed. Specifically, this procedure comprised the following steps (parts and percentages are by weight):

(a) Suds wash.Two (2) parts sodium metasilicate and one (1) part tallow soap were dispersed in 500 parts of hot water. Fabric, washed with moderate agitation for about one-half hour at about 80 C., then rinsed well in hot water.

(b) Bleach.To a solution of 0.1 part sodium metasilicate in water at about 68 C. about 2 parts bleach was added. (clorox, e.g.). The fabric was immersed and stirred well for about 8 minutes, then rinsed well in hot water.

Sour wash.-This solution comprised about 450 parts water, 18 parts of a solution of 2 grams per liter of a mixture comprising about 97.5% sodium silicofiuoride and 2.5% alkylbenzyl ammonium chloride and parts of a 10% solution of cetyl dimethylbenzyl ammonium chloride. The fabric was agitated for about 10 minutes in this solution at about 80 F., thereafter wrung dry and dried for about one hour at 300 F.

After each cycle, the fabric was checked for deterioration and failure. Deterioration and failure were indicated by the appearance of holes in the fabric and by loss of tensile strength to the point where the fabric could be pulled apart by hand. Stabilizer content was analyzed periodically during the laundering.

Example 1 A swatch of stereoregular polypropylene fabric was prepared as a control containing 0.38% (3.8 mg./g. of fabric) stabilizer, based on the weight of the polymer. The stabilizer was a phenolic-substituted triazine having the formula:

where R is an alkyl radical having about 9 carbon atoms.

Sections of this fabric were repeatedly subjected to the laundry cycle set forth above. After the fifth such cycle, holes had begun to develop in the fabric. After the sixth cycle, the fabric Was spotted with numerous widely scattered holes and could readily be pulled apart by hand pressure, indicating total failure of the specimen. Analysis for stabilizer revealed approximately 0.05% (0.5 m/ g. of fabric) remaining.

Another specimen of fabric was prepared of polypropylene fibers containing 0.125% (1.25 mg./g.) of the same stabilizer. This fabric was subjected to repeated laundry cycles the same as was the control, except that during each cycle 0.1 mg. of the phenolic triazine stabilizer per gram of fabric was added back to the fabric. This addition was accomplished by injecting a dilute solution of the stabilizer in ethyl alcohol into the sour rinse. After 10 cycles, the fabric showed very little evidence of deterioration and was still strong enough that it could not be pulled apart by hand.

This example clearly shows the improved laundering durability experienced when using the process of the invention. It also shows the greater efficiency of stabilizer utilization realized by the invention. The stabilizer content of the control decreased from 3.8 to 0.5 mg./ g. of

fabric, a consumption of 3.3 mg./ g. In treating the specimen according to the process of the invention, only 1.25 mg./ g. was used initially and 1.0 mg./ g. was added back, for a total of only 2.25 mg./ g. Moreover, considerably more of the stabilizer was left in the fabric at the end of the 10 laundry cycles.

Example 2 A sample of the same polypropylene fabric, containing 0.23% of the phenolic-substituted triazine was subjected to the laundry treatment described above. In this case, there was added to the sour wash, on each cycle, 0.4 mg./ g. of fabric of 2,2'-methylene bis(6-t-butyl-p-cresol). Addition was in the form of a solution of 1 gram of the bisphenol in 200 ml. of ethanol. After 11 cycles, the phenolic triazine stabilizer content was reduced to about 0.04%, but only one small hole had developed in the fabric, with no other indication of deterioration. The fabric could not be pulled apart by hand.

Example 3 When Example 2 was repeated, with the added bisphenol increased from 0.4 to 0.8 mg./g. of fabric, no evidence whatever of deterioration was seen after 11 laundry cycles.

Example 4 Fabrics of approximately 50% cotton and 50% polypropylene were prepared where the polypropylene portion contained about 0.08% of the phenolic-modified triazine stabilizer. These were subjected to laundering with 0.4 mg./ g. of the 2,2-methylene bis(6-t-butyl-p-cresol) stabilizer per gram of fabric being added to the sour Wash. The fabrics showed no signs of damage after 11 laundry cycles.

Example 4 is significant since polypropylene fiber is frequently used in blends with natural fibers such as cotton and wool. Thus, if the method of the invention is to have broad practical applicability, it must be usable with these blends, as well as with unblended fabrics. Also, it is frequently necessary to launder articles of different materials together. If the other fibers interfered in some way with the stabilizing efficiency of the polypropylene stabilizers, the method would likewise be of limited applicability. As the example shows, the method is at least as efiicient in the presence of cotton as in the presence of polypropylene exclusively.

Addition of the stabilizer to the system during laundering can be accomplished, as in the examples, by injecting the desired amount of stabilizer into the treating bath in the form of a dilute solution in alcohol. As the alcohol solution contacts the water in the bath, the stabilizer, being of very limited water solubility, immediately precipitates in the form of very fine particles which become dispersed throughout the bath. The finely dispersed form is most efiicient in the stabilizing effect which it can exert on the fabric, inasmuch as it dissolves more quickly as the material in solution becomes adsorbed on the surfaces of the fabric.

It is also possible to add the stabilizer to the bath in the form of finely divided powder. However, this method is less satisfactory since it is diflicult to prepare and isolate the stabilizer in such finely divided form. It is also more difficult to disperse the previously isolated powder uniformly in the water.

The method of the invention is advantageous as an eificient and inconspicuous way of maintaining the heat stability of polypropylene fabrics at a satisfactory level. Since the stabilizer is added with regular laundry chemicals, no special handling steps are required, i.e., no separate rinse to accomplish the stabilizer add-back. The invention is also advantageous in that it permits the use of simple, low cost stabilizers, rather than the more complex and, thus, more expensive stabilizers which must be used in order to ameliorate extractions or volatilization during laundering.

The preceding discussion has dealt with the application of the invention to commercial laundering, where the greatest need for it is experienced. However, the method can also be applied in home laundering, where a similar deteriorative effect on the fabric occurs, though at a lesser rate. In the case of home laundering, the additional stabilizer is added during the final water rinse step. If a fabric softener is employed, the stabilizer can be added with the softener.

What I claim and desire to protect by Letters Patent is:

1. A process for improving the stability to laundering of a fabric based on stereoregular polypropylene yarn which comprises (1) adding about 0.08 to 0.23% of a phenolic antioxidant during manufacture of the yarn and (2) adding about 0.01 to 0.08% of additional phenolic antioxidant during one or more steps of a conventional laundering process.

2. A process of claim 1 where the additional phenolic antioxidant is added to the final rinse of the laundering process.

3. The process of claim 2 when the initial phenolic antioxidant has the formula where R is an alkyl radical having 9 carbon atoms and the same antioxidant is added to the final rinse.

4. The process of claim 2 when the initial phenolic antioxidant is where R is an alkyl radical having 9 carbon atoms and 2,2'-methylene bis(6-t-butyl-p-cresol) is added to the final rinse.

References Cited WILLIAM D. MARTIN, Primary Examiner J. E. MILLER, Assistant Examiner U.S. Cl. X.R. 

